In the area of wireless communications, time division multiple access (TDMA) and code division multiple access (CDMA) protocols are used for communicating from a base station to a mobile station. The TDMA technology uses a single frequency for transmitting and receiving signals, while the CDMA systems use one frequency band for transmitting signals and another frequency band for receiving signals. In both cases, multipath can be a source of interference.
FIG. 1 is an example environment 100 in which multipath is typically present. The environment 100 includes a first antenna tower 105a and a second antenna tower 105b. Each antenna tower 105a, 105b has an associated base station (not shown). The environment 100 further includes a first office building 110a and a second office building 110b. In the first office building 110a, a subscriber unit 115 is within range of signals from both antenna towers 105a, 105b. 
There are several signaling paths from the antenna towers 105a, 105b to the subscriber unit 115. A first signaling path 120 is a direct signaling path from the first antenna tower 105a to the subscriber unit 115. A second signaling path 125 includes a reflection off the second office building 110b as the respective signal travels from the first antenna tower 105a to the subscriber unit 115. A third signaling path 130 is a direct signaling path from the second antenna tower 105b to the subscriber unit 115.
The first signaling path 120 is in the direction of the first antenna tower 105a. The subscriber unit 115 does not know where the first antenna tower 105a is located. The subscriber unit 115 can only point (i.e., direct a beam) in the direction of the strongest desired signal, if the subscriber signal is equipped with a steering antenna. The strongest desired signal is in the direction between the locations of the first antenna tower 105a and second office building 110b. 
In direction finding (DF), multipath tends to be harmful because it masks the true direction of the signal. The component of the multipath that is in-phase with the first signaling path 120 is actually helpful, and thus, the direction change is inconsequential. So, multipath is not all interference. However, the third signaling path 130 is all interference because it is not the same signal as being transmitted on the first signaling path and can never be in-phase with the signal on the first signaling path.
If the subscriber unit 115 employs a phased array antenna, it can use the phased array antenna to steer an associated antenna beam toward the first antenna tower 105a, or, in the case of multipath as just described, in the direction of the strongest desired signal. Additionally, the phased array antenna may be used to steer the associated antenna beam to receive signals from only the direct signaling path 120 from the first antenna tower 105a to remove the multipath effects (i.e., signal fading) caused by the second signal 125 or interference caused by the third signaling path 130.
FIG. 2 is a block diagram of the phased array antenna used by the subscriber unit 115 of FIG. 1 capable of steering the associated beam, where the steering is done by phase shifting the RF signals to/from the antenna elements composing the array antenna 200. The phased array antenna 200 is composed of antenna sub-assemblies 205. Each antenna sub-assembly 205 includes an antenna element 210, duplexer 215, and phase shifter 220. A control signal 225 is used to adjust the phase shifts imposed by each of the phase shifters 220.
In transmission mode, the sub-assemblies 205 of phased array antenna 200 receives a signal 230. The signal is phase shifted by the phase shifters 220 in a manner where, when the beams of all the antenna elements 210 are combined, the resulting effective beam (not shown) is directed as defined by the control signals 225. The signal 230 passes from the phase shifters 220 to the antenna elements 210 via the duplexes 215, which are in a transmit mode.
In receive mode, the antenna elements 210 receive RF signals most strongly from a direction defined by the same control signals 225. The antenna elements 210 provide the received signals to the duplexes 215, which are set in a receive mode to allow the received RF signal to pass to the phase shifters 220. The phase shifters 220 provide signals 230, which have been phase shifted, to a summer (not shown) to reconstruct the signal. The reconstructed signal is thereafter processed by a receiver (not shown).
Recently, experiments to determine optimal gain between a subscriber unit and antenna tower have shown that, when using transmission signals of different frequencies, the optimum signaling direction varies for the different frequencies. In CDMA technology, as defined for a subscriber unit, the receive (Rx) signals range between 1930-1990 MHz, and the transmission (Tx) signals span from 1850-1910 MHz. Further tests were conducted to determine whether the optimum signaling paths differ for the Tx and Rx signals of the CDMA technology, as in the case of transmitting signals having different frequencies. These further experiments proved that, in fact, the optimum signaling paths between a subscriber unit and base station antenna tower are frequency dependent, affecting signaling paths of Tx and Rx signals.
At least one reason for different optimum signaling directions for signals at different frequencies has been determined to be caused by different angles of refraction as the signals travel between the antenna tower and the subscriber unit antenna. For example, in CDMA technology, when the Tx and Rx signals travel through a glass of an office building window, the Tx signals xe2x80x9cbendxe2x80x9d at a first angle and the Rx signals xe2x80x9cbendxe2x80x9d at a second angle. The different angles of refraction may also result in the signals taking multiple paths inside an office in which the subscriber unit resides. Further, the Tx and Rx signals bend around objects external from the office building at different angles, which can be another source of difference in transmission paths. The net result of differences in angles and multipath is at best a reduction in signal-to-noise ratio (SNR) and at worst an interference causing disruption in communication.
In directional antenna technology, there is an assumption that the optimum directions of the signals traveling in the forward and reverse links are along the same path. Thus, once a direction has been selected, typically based on Rx signal-to-noise ratio (SNR), the selected direction is used for both Tx and Rx signals. While the selected direction may have been found to be optimal for one of the links, the selected direction of the antenna directivity may be sub-optimal for the other link, as learned during the experiments discussed above.
In general, the present invention provides a subscriber unit with an ability to transmit and receive signals in different directions simultaneously to allow for optimum gain in both directions. In this way, refraction and multipath effects resulting from communication signals operating at different frequencies can be compensated for to improve gain in both the forward and reverse links.
Accordingly, one embodiment of the present invention includes a directive antenna having plural antenna elements arranged in an antenna array. Frequency selective components are coupled to respective antenna elements, where the frequency selective components provide simultaneous frequency discrimination. At least two weighting structures are coupled to the frequency selective components to produce independently steerable beams having spectrally separated signals.
In an alternative embodiment, the present invention includes a directive antenna having plural antenna elements arranged in a parasitic antenna array. Frequency selective components are connected to a first subset of the antenna elements. Weighting structures are connected to a second subset of the antenna elements. The first and second subsets of antenna elements may be connected by a space-fed power distribution system to produce independently steerable beams having spectrally separated signals.
The frequency selective components may be designed to transmit and receive signals in, for example, a CDMA system in which the transmit and receive signaling bands are separated. The frequency selective components may also be designed to separate same direction signals having different frequencies. The frequency selective components may also separate more than two signals, in which case more than two phase-shifting elements are coupled to the frequency selective components. The frequency selective components may include a printed or non-printed technology, or combination thereof.
The weighting structures may include phase shifting elements to steer the beams independently. Independent control signals set-up respective phase shifts. The weighting structures may further include at least one variable gain amplifying component to independently amplify the signals received by or transmitted by the respective antenna elements. By having more than one variable gain amplifying component associated with each antenna element, the respective shapes of the beams can be optimized.
The directive antenna may further include a combiner associated with each beam being produced to combine signals transmitted or received by the antenna elements.
By having independently steerable and shapable beams, the directive antenna is attractive for use in a multi-band and/or multipath environment.
In one embodiment, the subscriber unit optimizes a forward link beam pattern (i.e., a receive, Rx, beam to receive signals in the forward link) based on a received pilot signal from a base station. The subscriber unit may also optimize the reverse (i.e., transmit, Tx) beam pattern based on a signal quality of a given received signal via a feedback metric from a base station over the forward link. Further, at the same time, the subscriber unit may steer the reverse beam (Tx beam) in the direction of maximum received power of a signal from a given base station, while optimizing the forward beam (Rx beam) on a best signal-to-noise ratio (SNR) or carrier-to-interference (C/I) level. These and other techniques for determining the direction of the beams in both forward and reverse links (i.e., receive and transmit beams, respectively, from the point of view of the subscriber unit) are provided in U.S. patent application Ser. No. 09/776,396 filed Feb. 2, 2001, entitled xe2x80x9cMethod and Apparatus for Performing Directional Re-Scan of an Adaptive Antenna,xe2x80x9d by Proctor et al, the entire teachings of which are incorporated herein by reference.